Methacrylic resins have excellent transparency, surface hardness, and so forth, and only display a small degree of the optical property of birefringence. For these reasons, methacrylic resin has been receiving attention as optical resin for optical material in various optical products such as flat panel displays, e.g. liquid crystal displays, plasma displays, and organic EL displays, small infrared sensors, fine optical waveguides, micro lenses, and DVD/Blu-ray Disc pickup lenses handling short-wavelength light, optical discs, optical films, and plastic substrates. The market for methacrylic resin is therefore expanding.
Particularly, methacrylic resin having a cyclic structure-containing main chain and compositions containing such methacrylic resin are known to have excellent performance in both heat resistance and optical property, and the demand for them has been rapidly growing year by year. However, in the case where methacrylic resin having a cyclic structure-containing main chain and exhibiting improved heat resistance and optical property is heat-molten to be processed into a film or other molded product to obtain a member for optical use, there is the drawback that, due to its high glass transition temperature, melt processing at a relatively high temperature needs to be performed, which tends to induce thermal decomposition. Besides, with the growing demand and the increasing requirement for higher quality in recent years, melt molding using a large extruder that has a higher discharge rate, includes various filters for foreign matter removal, generates heat easily, and has a relatively long residence time has been increasingly employed. This has raised expectations for the provision of methacrylic resin having excellent molding stability even under such harsh molding conditions and having excellent optical property and heat resistance enabling the provision of higher-quality members for optical use, and compositions containing such methacrylic resin.
Many compositions obtained by adding at least one antioxidant selected from antioxidants such as specific phenol-based antioxidant, phosphorus-based antioxidant, and sulfur-based antioxidant to methacrylic resin for the purposes of improving the heat stability of the methacrylic resin and suppressing color change due to heat melting, and molded products and films using such compositions have conventionally been known.
Moreover, regarding methacrylic resin having a ring structure in a main chain and having a high glass transition temperature and compositions containing such methacrylic resin, the following attempts have been made, in addition to the above-mentioned purposes; adding an organic phosphorous compound including a phosphorus-based antioxidant as a stabilizer for coloring inhibition of a polymerization product, and for cyclization catalysis and/or intermolecular crosslinking inhibition when using a production method of introducing a ring structure into a main chain by cyclization reaction; and adding the antioxidant in any of the steps for manufacturing the resin composition to improve heat stability.
For example, JP H6-116331 A (PTL 1) proposes a composition containing 10 wt % or more of a maleimide-based monomer and also containing, with respect to the whole monomers, 0.001 wt % to 1 wt % of a compound having a phosphorus atom. According to PTL 1, such a composition can exhibit high transparency without yellowing even when exposed to high temperature for a long time.
JP H8-217944 A (PTL 2) proposes a resin composition containing a hindered amine-based light stabilizer, a hindered phenol-based antioxidant, and a phosphite-based antioxidant in a copolymer containing 10 wt % to 70 wt % of methyl methacrylate, 5 wt % to 30 wt % of N-substituted maleimide, 15 wt % to 85 wt % of cyclohexyl methacrylate, and 0 wt % to 30 wt % of other copolymerizable monomer. PTL 2 proposes obtaining a molded product having little foreign matter contamination due to continuous long-time molding and little coloring during molding and little coloring in a high-temperature atmosphere (around 100° C.), and excellent heat resistance and low birefringence.
JP H10-45850 A (PTL 3) proposes a method of efficiently manufacturing resin with little coloring by employing a polymerization method whereby, when polymerizing a radical-polymerizable monomer containing N-substituted maleimide, at least one antioxidant selected from the group consisting of a phenol-based antioxidant and a phosphorus-based antioxidant is used in such a manner that part of the antioxidant is caused to coexist in the polymerization of the monomer component and the rest of the antioxidant is added after the polymerization of the monomer component ends.
JP 2008-76764 A (PTL 4) proposes a composition for molding containing: a resin component including, as a main component, methacrylic resin having a lactone ring structure; and an antioxidant selected from a phenol-based antioxidant, a thioether-based antioxidant, and a phosphorus-based antioxidant that is 0.02 parts by weight or more with respect to 100 parts by weight of the resin component and whose weight decrease in heating at 300° C. for 20 min is 10% or less. According to PTL 4, such a composition has excellent heat resistance and excellent optical transparency, and can suppress foaming in an optical film even when the molding temperature is 250° C. or more.
JP 2013-83907 A (PTL 5) proposes a method of including an antioxidant such as a phosphorus-based antioxidant in a resin composition containing: methacrylic resin having a cyclic structure-containing main chain; and elastic organic fine particles having a conjugated diene monomer structural unit as an essential component, thus improving the stability of the optical property of a retardation film obtained by molding. According to PTL 5, the antioxidant may be mixed together when mixing the methacrylic resin and the elastic organic fine particles, e.g. by kneading using a twin screw extruder, or, when preparing the methacrylic resin or the elastic organic fine particles, mixed together with the corresponding structural monomer component.